This invention relates to means for determining the amount of solid carbon dioxide (CO.sub.2) in a container used as the source of refrigeration in trucks, trailers, rail cars and like chambers in which perishable products such as frozen foods are transported.
U.S. Pat. No. 3,561,226 to Julius Rubin discloses such a refrigeration system based on a container which is filled with CO.sub.2 snow and is mounted under the roof of the truck or rail car. This refrigeration system has found favor particularly in the railroad shipment of food products.
Often an appreciable potion of the CO.sub.2 snow charged into the container, also referred to as the cold box, for a particular trip lasting several days will remain in the cold box after the rail car has reached the delivery depot and has been emptied of the goods it transported. Hence, the rail car is ready to be loaded again with perishable products for shipment to another destination. However, no simple and satisfactory way has heretobefore been devised for estimating the amount of CO.sub.2 snow still in the cold box after the last delivery. Perhaps, there is enough CO.sub.2 snow for the next three-day trip that charging the cold box with more CO.sub.2 snow is unnecessary and wasteful. However, there is the danger that the residual CO.sub.2 snow will suffice for only a day. Hence, in the absence of a reasonable estimate of the CO.sub.2 snow still in the cold box, it is common practice to be overcautious by charging additional CO.sub.2 snow into the cold box until it is full each time the rail car is to be reloaded with goods requiring refrigeration during the next shipment. This is obviously a wasteful practice.
A principal object of this invention is to provide simple means for estimating the amount of CO.sub.2 snow in a container or cold box of a refrigeration system of the type disclosed in U.S. Pat. No. 3,561,226.